protected override void OnRender(DrawingContext dc)
{
Rect rect = new Rect(new Point(0,0), RenderSize);
rect.Inflate(-1, -1);
Pen pen = new Pen(SystemColors.HighlightBrush, 1);
if (IsHighlighted)
dc.DrawRectangle(SystemColors.ControlDarkBrush, pen, rect);
else if (IsSelected)
dc.DrawRectangle(SystemColors.ControlLightBrush, pen, rect);
else
dc.DrawRectangle(Brushes.Transparent, null, rect);
}